Conventionally, a laser beam machine has been employed for cutting various kinds of materials. In most cases, the laser beam machine is employed for cutting off a workpiece by irradiating a laser beam perpendicularly to a surface of the workpiece. The cutting of the workpiece is carried out by irradiating the laser beam while blowing an assist gas to the cutting point. During the cutting, the direction of irradiation of the laser beam and the direction of blowing of the assist gas are made coincident with each other.
In welding two steel plates by holding them abutting on each other at ends thereof, each of the ends for welding is subjected to so-called edge preparation in advance, and then faces of the ends beveled by the edge preparation are caused to abut on each other to weld the two steel plates. To effect the edge preparation by using the laser beam, the laser beam is irradiated diagonally to the surface of each steel plate.
However, even if the laser beam is irradiated diagonally to the steel plate, the laser beam only shaves off the surface of the steel plate but cannot diagonally cut off the steel plate. Particularly, it is very difficult to diagonally cut off steel plates having a thickness of 6 mm or larger.
FIG. 3 illustrates an example of diagonal cutting by the conventional method described above. In the figure, a laser beam condensed by a condensing lens 102 is emitted via a machining nozzle 101 onto a steel plate 5, while an assist gas introduced via an assist gas-introducing port 103 is blown via the machining nozzle 101 against the steel plate 5. The direction of irradiation of the laser beam and the direction of blowing of the assist gas are thus made coincident toward the steel plate 5, by the machining nozzle 101. As described above, if the direction of irradiation of the laser beam and the direction of blowing of the assist gas are coincident with each other, and both of them are caused to impinge on the steel plate 5 in a diagonal direction, the laser beam cannot enter the steel plate deeply in the diagonal direction, but ends in shaving off the surface of the steel plate 5. This is because both the laser beam and the assist gas are reflected from the surface of the steel plate 5. Therefore, it has been difficult to perform the edge preparation of the steel plate 5 by the use of the laser beam.